Air pollution control device and method for reducing amount of mercury in flue gas

ABSTRACT

The air pollution control device includes: NH 4 Cl solution supply means  16  for spraying an NH 4 Cl solution  14  by a plurality of spray nozzles  15  into a flue gas duct  13  at the downstream of the boiler  11 ; a reduction-denitration device  18  including a denitration catalyst for reducing NOx in the flue gas  12  with NH 3  and for oxidizing Hg under the coexistence with HCl; and a wet desulfurization device  22  for reducing the amount of Hg oxidized in the reduction-denitration device  18  by using a limestone-gypsum slurry  21 . The NH 4 Cl solution supply means  16  supplies the NH 4 Cl solution  14  from the spray nozzles  15  so as to prevent the NH 4 Cl solution  14  from being adhered to an inner wall of the flue gas duct  13  through which the flue gas  12  is flowing.

FIELD

The present invention relates to an air pollution control device for performing an oxidation treatment of mercury contained in a flue gas discharged from a boiler or the like, and a method for reducing the amount of mercury in a flue gas.

BACKGROUND

A coal combustion flue gas or flue gas generated when burning heavy oil sometimes contains metallic mercury) (Hg⁰) in addition to soot and dust, sulfur oxides (SOx), and nitrogen oxides (NOx). In recent years, various methods and devices for treating this metallic mercury have been devised in combination with a denitration device for reducing NOx and a wet desulfurization device using an alkali absorbent as an SOx absorbent.

As a method for treating metallic mercury in a flue gas, there has been suggested a system in which an ammonium (NH₃) solution is sprayed in a flue gas duct on an upstream side of a high-temperature denitration device so as to perform reduction and denitration; an oxidation auxiliary agent such as a hydrochloric acid (HCl) solution is also sprayed so as to oxidize (chlorinate) mercury on a denitration catalyst to obtain a water-soluble mercury chloride; and the amount of mercury is then reduced by a wet desulfurization device disposed on a downstream side (for example, see Patent Literature 1).

Moreover, as a method for supplying HCl, there has been a method in which a hydrochloric acid (HCl) solution is vaporized using a hydrogen chloride (HCl) carburetor to obtain a hydrogen chloride (HCl) gas; and after adjusting it to a mixed gas containing HCl with a predetermined concentration, the mixed gas is dispersed into a flue gas duct so as to be sprayed evenly into a flue gas containing mercury (for example, see Patent Literature 2).

Moreover, as another method for supplying HCl, there has been a method in which powdered ammonium chloride (NH₄Cl) is added in a flue gas duct on an upstream side of a denitration device; NH₄Cl is sublimed by a high ambient temperature of a flue gas, so that HCl and ammonia (NH₃) are gasified respectively; and the gasified HCl gas and NH₃ gas are mixed with the flue gas (for example, see Patent Literature 3).

With the methods for treating metallic mercury in a flue gas as described above, when a hydrochloric acid solution is used, there is a problem that great care and cost are required in its transportation, handling, and the like since hydrochloric acid is a dangerous substance. Moreover, when the HCl carburetor is used, steam or the like is required as a heat source. Thus, there is a problem that costs for the facility, operation, maintenance, and the like of the HCl carburetor and the like are required. Furthermore, when NH₄Cl powders are used, a particle diameter thereof needs to be reduced for the dispersion thereof. Thus, there is a problem that the handling thereof is difficult and control for the sprayed amount thereof is not easy.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No. Hei.     10-230137 -   Patent Literature 2: Japanese Patent Application Laid-Open No.     2007-167743 -   Patent Literature 3: Japanese Patent Application Laid-Open No.     2008-221087

SUMMARY Technical Problem

In view of this, in order to oxidize Hg⁰ with a denitration catalyst, there has been studied a method for spraying an ammonium chloride (NH₄Cl) solution on an upstream side of a denitration device in recent years. As compared to the method using a hydrochloric acid solution as in the conventional technique, dangerousness of the NH₄Cl solution is small, and the transportation and handling thereof are therefore easy. Furthermore, since there is no need for the facility such as a carburetor for spraying a liquid, the cost can be reduced.

A schematic diagram of an air pollution control system for the flue gas discharged from a boiler is shown in FIG. 29. As shown in FIG. 29, an air pollution control system 100 includes: an NH₄Cl spray device 105 for spraying an NH₄Cl solution 103 into the flue gas 102 in a flue gas duct 104, the flue gas 102 containing NOx and Hg⁰ discharged from a boiler 101 for supplying coal as a fuel; a reduction-denitration device 106 including a denitration catalyst for reducing NOx and oxidizing Hg⁰; and a desulfurization device 107 for reducing the amount of oxidized HgCl in the flue gas 102. The NH₄Cl solution 103 is sprayed from an NH₄Cl solution tank 108 into the flue gas 102 discharged from the boiler 101 by spray nozzles 109. The NH₄Cl solution 103 is gasified, so that an NH₃ gas and an HCl gas are mixed with the flue gas 102. Thereafter, the flue gas 102 is supplied to the reduction-denitration device 106 to perform NOx reduction and Hg⁰ oxidation by a denitration catalyst in the reduction-denitration device 106. Then, the flue gas 102 after the removal of NOx is heat-exchanged with air 111 by an air preheater (air heater) 110 to recover heat. Then, the flue gas 102 is supplied to an electronic precipitator 112, and soot and dust in the flue gas 102 after the heat recovery are removed. The flue gas 102 is supplied to the desulfurization device 107, brought into gas-liquid contact with a gypsum stone slurry 113 which is supplied to the desulfurization device 107 to reduce the amounts of SOx and Hg, and discharged to the outside as a purged gas 114 from a stack 115.

Moreover, an NOx concentration in the flue gas 102 is measured by an NOx meter 116 disposed on the upstream side of the reduction-denitration device 106 in the flue gas duct 104, and an Hg concentration is measured by an Hg concentration meter 117 disposed on the downstream side of the desulfurization device 107. Based on the measured values of the measured NOx concentration and Hg concentration, a supply amount and a concentration of the NH₄Cl solution 103 to be supplied from the NH₄Cl solution tank 108 are calculated by an arithmetic unit 118. Based on the calculated supply amount and concentration of the NH₄Cl solution 103, a supply amount of the NH₄Cl solution 103 to be supplied into the flue gas duct 104 is controlled by control means 119.

Moreover, an oxidation-reduction potential is measured by an oxidation-reduction potential measurement device 120 disposed at a bottom portion of the desulfurization device 107 and a supply amount of air 121 is adjusted, thereby preventing the reduction and spread of the mercury oxide.

As described above, the NH₄Cl solution 103 is supplied into the flue gas 102, and accordingly, the amount of NOx in the flue gas 102 can be reduced and Hg can be oxidized.

Here, according to the air pollution control system 100 shown in FIG. 29, a plurality of spray nozzles 109 are provided along the wall surface of the flue gas duct 104, for example, as shown in FIG. 30, and the smaller the number of the spray nozzles is, the more preferable it is in view of the installation cost and the maintenance performance thereof. However, if the number of spray nozzles is too small, the NH₄Cl solution 103 cannot be evenly sprayed into the flue gas duct 104. Thus, there is a problem that NH₃ and HCl generated from the NH₄Cl solution 103 cannot be supplied evenly into the flue gas 102, resulting in uneven NH₃ concentration and HCl concentration in the flue gas 102 and therefore a reduction in the denitration ability and the mercury oxidation ability.

Moreover, if the NH₄Cl solution 103 is adhered to the wall surface of the flue gas duct 104 before the vaporization thereof, there is a problem that corrosion of the flue gas duct 104, deposition of ash, a breakage due to heat shock, and the like may occur.

In view of the above-described problems, an object of the present invention is to provide an air pollution control device capable of evenly supplying a reducing agent and a mercury chlorinating agent into a flue gas duct without concentration unevenness and capable of maintaining the mercury removal ability and the nitrogen oxide reducing ability, and a method for reducing the amount of mercury in a flue gas.

Solution to Problem

In order to solve the above-described problems, the following configurations can be employed.

1) According to an aspect of the present invention, an air pollution control device for reducing amounts of a nitrogen oxide and mercury contained in a flue gas from a boiler, includes: a reduction-oxidation auxiliary agent supply unit for spraying in a liquid state a reduction-oxidation auxiliary agent that produces an oxidizing gas and a reducing gas upon gasification thereof, into a flue gas duct at a downstream of the boiler by a plurality of spray nozzles; a reduction-denitration unit including a denitration catalyst for reducing a nitrogen oxide in the flue gas with the reducing gas and for oxidizing mercury under coexistence with the oxidizing gas; and a wet desulfurization unit for reducing the amount of mercury oxidized in the reduction-denitration unit using an alkali absorbent. The spray nozzles supply the reduction-oxidation auxiliary agent into the flue gas duct so as to prevent the reduction-oxidation auxiliary agent from being adhered to an inner wall of the flue gas duct.

2) Advantageously, in the air pollution control device, based on a distance 1 required for droplet gasification, which is obtained at least from a gas flow velocity, a droplet initial velocity, a droplet diameter, a flue gas temperature and a droplet temperature, and a jet angle α, the spray nozzles are disposed so that a shortest distance x from a nozzle hole of the spray nozzle to the inner wall of the flue gas duct satisfies the following expression:

x>1×sin α  (1).

3) Advantageously, in the air pollution control device, the reduction-oxidation auxiliary agent is ammonium chloride.

4) Advantageously, in the air pollution control device, a nozzle hole of the spray nozzle is provided at a position 0.5 m or longer away from a wall surface of the flue gas duct.

5) Advantageously, in the air pollution control device, nozzle holes of the plurality of spray nozzles are disposed so as to satisfy the following expression:

a≦b/5  (2)

where a denotes a distance between the spray nozzles, and b denotes a long side length out of lengths of a cross-section of the flue gas duct.

6) Advantageously, in the air pollution control device, the spray nozzle has a plurality of nozzle holes for spraying the reduction-oxidation auxiliary agent.

7) Advantageously, in the air pollution control device, a distance between the nozzle holes is 0.3 m or shorter.

8) Advantageously, in the air pollution control device, respective sprayed amounts from the spray nozzles can be changed.

9) Advantageously, in the air pollution control device, further includes one of or both of an ammonia gas supply unit for supplying an ammonia gas into the flue gas duct and a hydrogen chloride gas supply unit for supplying a hydrogen chloride gas into the flue gas duct, which are provided between the reduction-oxidation auxiliary agent supply unit and the reduction-denitration unit.

10) According to another aspect of the present invention, a flue gas mercury reducing method for reducing amounts of a nitrogen oxide and mercury contained in a flue gas from a boiler, includes: a reduction-oxidation auxiliary agent supplying step of spraying, in a liquid state, a reduction-oxidation auxiliary agent, which produces an oxidizing gas and a reducing gas upon gasification thereof, into a flue gas duct of the boiler by a plurality of spray nozzles; a reduction-denitration treatment step of reducing a nitrogen oxide in the flue gas with the reducing gas and for oxidizing mercury under coexistence with the oxidizing gas using a denitration catalyst; and a wet desulfurization step of reducing an amount of mercury oxidized in the reduction-denitration treatment step using an alkali absorbent. The spray nozzles supply the reduction-oxidation auxiliary agent into the flue gas duct so as to prevent the reduction-oxidation auxiliary agent from being adhered to an inner wall of the flue gas duct.

11) Advantageously, in the flue gas mercury reducing method, based on a distance 1 required for droplet gasification, which is obtained at least from a gas flow velocity, a droplet initial velocity, a droplet diameter, a flue gas temperature, and a droplet temperature, and a jet angle α, the spray nozzles are disposed so that a shortest distance x from the nozzle hole of the spray nozzle to the inner wall of the flue gas duct satisfies the following expression:

x>1×sin α  (3).

12) Advantageously, in the flue gas mercury reducing method, the reduction-oxidation auxiliary agent is ammonium chloride.

13) Advantageously, in the flue gas mercury reducing method, a nozzle hole of the spray nozzle is provided at a position 0.5 m or longer away from a wall surface of the flue gas duct.

14) Advantageously, in the flue gas mercury reducing method, nozzle holes of the plurality of spray nozzles are disposed so as to satisfy the following expression:

a≦b/5  (4)

where a denotes a distance between the spray nozzles, and b denotes a long side length out of lengths of a cross-section of the flue gas duct.

15) Advantageously, the flue gas mercury reducing method includes a flow rate measurement step of measuring a flow velocity of the flue gas on an upstream side of a supply position at which the reduction-oxidation auxiliary agent is supplied. A sprayed amount, a spray angle, and an initial velocity of the reduction-oxidation auxiliary agent are adjusted based on the measured flow velocity of the flue gas.

16) Advantageously, the flue gas mercury reducing method includes: a nitrogen oxide concentration measurement step of measuring a nitrogen oxide concentration in the flue gas on a pre-step side of the reduction-denitration treatment step; and a mercury concentration measurement step of measuring a mercury concentration in the flue gas on a post-step side of the reduction-denitration treatment step. Based on one of or both of the nitrogen oxide concentration in the flue gas obtained by the nitrogen oxide concentration measurement step and the mercury concentration in the flue gas obtained by the mercury concentration measurement step, a supply amount of the reduction-oxidation auxiliary agent to be supplied in the reduction-oxidation auxiliary agent supplying step is adjusted.

17) Advantageously, the flue gas mercury reducing method includes: one of or both of an ammonia gas supplying step of supplying an ammonia gas into the flue gas duct and a hydrogen chloride gas supplying step of supplying a hydrogen chloride gas into the flue gas duct between the reduction-oxidation auxiliary agent supplying step and the reduction-denitration treatment step. A sprayed amount, a spray angle, and an initial velocity of one of or both of the ammonia gas supplied by the ammonia gas supplying step and the hydrogen chloride gas supplied by the hydrogen chloride gas supplying step are adjusted based on the flow velocity of the flue gas measured by the flow rate measurement step.

To further solve the above-described problems, the following configurations can be further employed.

18) That is, mixing means for promoting mixing of the oxidizing gas and the reducing gas produced upon the gasification of the reduction-oxidation auxiliary agent with the flue gas may be provided on a downstream side of a region where the reduction-oxidation auxiliary agent is gasified.

19) The mixing means may be provided in the flue gas duct at 1 m or more and 10 m or less downstream side of the supply position at which the reduction-oxidation auxiliary agent is supplied.

20) The mixing means may be formed by a unit in which a plurality of swirling flow inductive members for generating a swirling flow in the flue gas are provided so as to be perpendicular to a flow direction of the flue gas.

21) The mixing means may be formed by providing a plurality of the units in the flow direction of the flue gas.

22) The swirling flow inductive member may include: a pair of first swirling flow inductive plates having opposed surfaces on an inlet side of the flue gas; and a pair of second swirling flow inductive plates having opposed surfaces on a discharge side of the flue gas. At a connecting portion for connecting the first swirling flow inductive plate and the second swirling flow inductive plate, the first swirling flow inductive plate and the second swirling flow inductive plate may be connected thereto in such a way that the opposed surfaces thereof are directed differently.

23) A width L and a height D of the swirling flow inductive member may be within ranges of the following expressions.

MIN(B,H)/10≦L≦MIN(B,H)  (5)

MIN(B,H)/10≦D≦5×MIN(B,H)  (6)

Note however that B denotes a length of one side in a cross-section of the flue gas duct at an installation position thereof; H denotes a length of the other side in the cross-section of the flue gas duct; and MIN(B,H) denotes a value of a length of a shorter side out of the length B of the one side in the cross-section of the flue gas duct and the length H of the other side in the cross-section of the flue gas duct.

24) The flue gas duct may include a protruding member provided on the inner wall of the flue gas duct on the downstream side of the supply position at which the reduction-oxidation auxiliary agent is supplied into the flue gas duct.

25) The flue gas duct may include a narrowed portion for narrowing a passage in the flue gas duct on the downstream side of the supply position at which the reduction-oxidation auxiliary agent is supplied into the flue gas duct.

26) A guide vane provided on the upstream side of the reduction-denitration means may be provided with a mixing promoting auxiliary member for promoting the mixing of the gasified reducing gas and oxidizing gas into the flue gas.

27) The spray nozzle may be a two-fluid nozzle for jetting the reduction-oxidation auxiliary agent and air for spraying the reduction-oxidation auxiliary agent.

28) A flow rate measurement device for measuring a flow velocity of the flue gas may be provided on the upstream side of the supply position at which the reduction-oxidation auxiliary agent is supplied.

Advantageous Effects of Invention

According to the present invention, by spraying in a liquid state a reduction-oxidation auxiliary agent, which produces an oxidizing gas and a reducing gas upon gasification thereof, into a flue gas duct at a downstream of a boiler by using a plurality of spray nozzles and supplying the reduction-oxidation auxiliary agent into the flue gas duct by the spray nozzles while preventing the reduction-oxidation auxiliary agent from being adhered to the inner wall of the flue gas duct, the oxidizing gas and the reducing gas, which are generated when the reduction-oxidation auxiliary agent is gasified, can be supplied evenly in the flue gas duct without concentration unevenness. Thus, it is possible to improve a mercury oxidation ability and improve a nitrogen oxide reducing ability in the reduction-denitration device.

Moreover, since it is possible to prevent the reduction-oxidation auxiliary agent from being adhered to the wall surface of the flue gas duct before the gasification thereof, it is possible to prevent a breakage of the flue gas duct resulting from corrosion of the flue gas duct or the like from occurring.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the configuration of an air pollution control device according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a jet angle of an NH₄Cl solution sprayed from a spray nozzle with respect to a flue gas duct.

FIG. 3 is a diagram showing an example of the configuration of NH₄Cl solution supply means.

FIG. 4 is a diagram concretely showing the configuration of a denitration device.

FIG. 5 is a diagram showing a cross-section of a flue gas duct of an air pollution control device according to a second embodiment of the present invention as viewed from a flow direction of the flue gas.

FIG. 6 is a diagram showing a cross-section of a flue gas duct of an air pollution control device according to a third embodiment of the present invention as viewed from a flow direction of the flue gas.

FIG. 7 is a diagram showing a configuration of a spray nozzle in a simplified manner.

FIG. 8 is a partial enlarged view of the spray nozzle.

FIG. 9 is a diagram showing a cross-section of a flue gas duct of an air pollution control device according to a fourth embodiment of the present invention as viewed from a flow direction of the flue gas.

FIG. 10 is a diagram showing a configuration of an air pollution control device according to a fifth embodiment of the present invention in a simplified manner.

FIG. 11 is a plan view showing an example of a mixer.

FIG. 12 is a plan view of a swirling flow inductive member forming the mixer.

FIG. 13 is a front view of the swirling flow inductive member.

FIG. 14 is a perspective view of the swirling flow inductive member.

FIG. 15 is a diagram schematically showing a gas flow of a flue gas when the mixer is installed within the flue gas duct.

FIG. 16 is a partial enlarged view of FIG. 15.

FIG. 17 is a diagram schematically showing an example of the NH₃ gas concentration distribution in the flue gas when the mixer is not installed within the flue gas duct.

FIG. 18 is a diagram schematically showing an example of the NH₃ gas concentration distribution in the flue gas when the mixer is installed within the flue gas duct.

FIG. 19 is a diagram showing the relationship between a pressure loss of the mixer and a dimension of the mixer.

FIG. 20 is a diagram showing a flue gas duct of an air pollution control device according to a sixth embodiment of the present invention as viewed from a short side direction thereof.

FIG. 21 is a diagram showing the flue gas duct as viewed from a long side direction thereof.

FIG. 22 is a diagram showing a flue gas duct of an air pollution control device according to a seventh embodiment of the present invention as viewed from a short side direction thereof.

FIG. 23 is a diagram showing the flue gas duct as viewed from a long side direction thereof.

FIG. 24 is a diagram showing the flue gas duct as viewed from the short side direction thereof.

FIG. 25 is a diagram showing the flue gas duct as viewed from the long side direction thereof.

FIG. 26 is a diagram showing part of an air pollution control device according to an eighth embodiment of the present invention.

FIG. 27 is a partial enlarged view of part in FIG. 26.

FIG. 28 is a diagram showing a configuration of an air pollution control device according to a ninth embodiment of the present invention in a simplified manner.

FIG. 29 is a diagram showing a schematic diagram of an air pollution control system for the flue gas discharged from a boiler.

FIG. 30 is a diagram showing an arrangement of spray nozzles as viewed from a flow direction of the flue gas in a flue gas duct.

DESCRIPTION OF EMBODIMENTS

The present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited by those embodiments. Moreover, constituent elements in the following embodiments include those which can be conceived easily by those skilled in the art or those substantially the same.

First Embodiment

An air pollution control device according to the first embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram showing the configuration of the air pollution control device according to the first embodiment of the present invention.

As shown in FIG. 1, an air pollution control device 10 according to the present embodiment is an air pollution control device for reducing the amounts of nitrogen oxide (NOx) and mercury (Hg) contained in flue gas 12 from a boiler 11. The device 10 includes: ammonium chloride (NH₄Cl) solution supply means (reduction-oxidation auxiliary agent supply means) 16 for spraying, in a liquid state, an ammonium chloride (NH₄Cl) solution 14 containing ammonium chloride (NH₄Cl) as a reduction-oxidation auxiliary agent by a plurality of spray nozzles 15 in a flue gas duct 13 at the downstream of the boiler 11; a reduction-denitration device (reduction-denitration means) 18 including a denitration catalyst for reducing NOx in the flue gas 12 using an ammonia (NH₃) gas as a reducing gas and for oxidizing Hg under the coexistence with a hydrogen chloride (HCl) gas used as an oxidizing gas; a heat exchanger (air heater) 19 for performing heat-exchange of the denitrated flue gas 12; a precipitator 20 for reducing the amounts of soot and dust in the denitrated flue gas 12; and a wet desulfurization device 22 for reducing the amount of the Hg oxidized in the reduction-denitration device 18 using a limestone-gypsum slurry 21 as an alkali absorbent.

Note that although NH₄Cl is used as a reduction-oxidation auxiliary agent in the air pollution control device 10 according to the present embodiment, the present invention is not limited thereto. Any reduction-oxidation auxiliary agent can be used as long as it generates an oxidizing gas and a reducing gas when gasified.

Moreover, the reduction-oxidation auxiliary agent used in the present invention refers to one functioning as an oxidation auxiliary agent used for oxidizing and chlorinating mercury (Hg) under the coexistence with an oxidizing gas and as a reducing agent for reducing NOx by a reducing gas. In the present embodiment, the HCl gas is used as the oxidizing gas, and the NH₃ gas is used as the reducing gas.

The NH₄Cl solution 14 is supplied to the flue gas 12 discharged from the boiler 11 by the NH₄Cl solution supply means 16. The NH₄Cl solution supply means 16 includes the spray nozzle 15. The spray nozzle 15 includes: an ammonium chloride (NH₄Cl) solution supply pipe 25 for supplying the NH₄Cl solution 14 in a liquid state into the flue gas duct 13; and an air supply pipe 27 for supplying, into the flue gas duct 13, air 26 for compressing and spraying the NH₄Cl solution 14 into the flue gas duct 13, and is composed of a two-fluid nozzle. The spray nozzle 15 includes nozzle holes 15 a for simultaneously jetting the NH₄Cl solution 14 and the air 26 at tip portions of the ammonium chloride (NH₄Cl) solution supply pipe 25 and the air supply pipe 27.

The NH₄Cl solution 14 is adjusted within an ammonium chloride (NH₄Cl) solution tank 28 to have a predetermined concentration. Moreover, a flow rate of the NH₄Cl solution 14 supplied from the NH₄Cl solution supply pipe 25 is adjusted by a valve V1. The NH₄Cl solution 14 passes through the NH₄Cl solution supply pipe 25 from the NH₄Cl solution tank 28 and is sprayed into the flue gas duct 13 from the spray nozzle 15.

The NH₄Cl solution supply means 16 includes the spray nozzle 15 disposed in such a way that the NH₄Cl solution 14 is supplied so as to prevent the NH₄Cl solution 14 from being adhered to an inner wall 13 a (see FIG. 2) of the flue gas duct 13 through which the flue gas 12 is flowing. A preferred arrangement for supplying the NH₄Cl solution 14 while preventing the NH₄Cl solution 14 from being adhered to the inner wall 13 a of the flue gas duct 13 through which the flue gas 12 flows is a configuration in which the spray nozzle 15 is disposed inside the flue gas duct 13 with a certain distance or longer away from the inner wall 13 a of the flue gas duct 13. A certain distance or longer refers to a distance sufficient for a droplet of the sprayed NH₄Cl solution 14 to be gasified before reaching to the inner wall 13 a of the flue gas duct 13 from the nozzle hole of the spray nozzle 15. In view of the actual dimension of the flue gas duct and the actual treatment conditions, it is preferable that the nozzle hole of the spray nozzle 15 be provided at a position 0.5 m or longer away from a wall surface of the flue gas duct 13, for example.

The reason why the position of the nozzle hole of the spray nozzle 15 is set to the position 0.5 m or longer away from the wall surface of the flue gas duct 13 is that it is necessary to take into consideration a gas flow velocity of the flue gas 12, an initial velocity of a droplet of the NH₄Cl solution 14 sprayed from the spray nozzle 15, a droplet diameter, a jet angle of the NH₄Cl solution 14 sprayed from the spray nozzle 15 with respect to the flue gas duct 13, a flue gas temperature of the flue gas 12, a droplet temperature of the NH₄Cl solution 14, and the like. In one example thereof, it can be specifically determined as follows.

That is, when the gas flow velocity of the flue gas 12 inside the flue gas duct 13 is about 15 m/s, the initial velocity of the droplet of the NH₄Cl solution 14 sprayed from the spray nozzle 15 is about 300 m/s, the gas temperature of the flue gas 12 is about 350° C., and the droplet temperature of the NH₄Cl solution 14 is about 20° C., an estimated amount of time it takes for a droplet of the NH₄Cl solution 14 to be evaporated after being sprayed and an estimated moved distance over which a droplet of the NH₄Cl solution 14 is moved before the evaporation thereof vary depending on the droplet diameter of the NH₄Cl solution 14.

Examples of a relationship among a droplet diameter of the NH₄Cl solution 14, an amount of time it takes for a droplet to be evaporated after being sprayed, and a moved distance over which a droplet is moved before the evaporation thereof are shown in Table 1. In Table 1, t represents an amount of time it takes for a droplet of the NH₄Cl solution 14 to be evaporated after being sprayed, and 1 represents a moved distance over which a droplet is moved before the evaporation thereof.

TABLE 1 AMOUNT OF TIME MOVE DISTANCE OVER DROPLET FROM SPRAY TO WHICH DROPLET MOVES DIAMETER EVAPORATION BEFORE EVAPORATION (μm) t (s) l (m) 40 0.032 0.76 60 0.068 1.6 80 0.119 2.7

As shown in Table 1, when the droplet diameter of the NH₄Cl solution 14 is about 40 μm, the amount of time t it takes for this droplet to be evaporated after being sprayed is about 0.032 s, and the moved distance 1 over which the droplet of the NH₄Cl solution 14 is moved before the evaporation thereof is calculated to be about 0.76 m from the spray nozzle 15 in a direction parallel to the flow of the flue gas 12. Also, when the droplet diameter of the NH₄Cl solution 14 is about 60 μm, the amount of time t it takes for this droplet to be evaporated after being sprayed is about 0.068 s, and the moved distance 1 over which the droplet of the NH₄Cl solution 14 is moved before the evaporation thereof is calculated to be about 1.6 m from the spray nozzle 15 in the direction parallel to the flow of the flue gas 12. Also, when the droplet diameter of the NH₄Cl solution 14 is about 80 μm, the amount of time t it takes for this droplet to be evaporated after being sprayed is about 0.119 s, and the moved distance 1 over which the droplet of the NH₄Cl solution 14 is moved before the evaporation thereof is calculated to be about 2.7 m from the spray nozzle 15 in the spraying direction.

Next, a jet angle of the NH₄Cl solution 14 sprayed from the spray nozzle 15 with respect to the flue gas duct 13 will be examined. FIG. 2 is a diagram illustrating a jet angle of the NH₄Cl solution sprayed from the spray nozzle with respect to the flue gas duct. In FIG. 2, α denotes a jet angle of a droplet of the NH₄Cl solution 14 sprayed from a nozzle hole 15 a of the spray nozzle 15 with respect to a wall surface of the flue gas duct 13, and x denotes a shortest distance from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15.

As shown in FIG. 2, the spray nozzle 15 is disposed so as to satisfy the following expression (1) in accordance with the jet angle α of a droplet of the NH₄Cl solution 14 sprayed from the nozzle hole 15 a of the spray nozzle 15 with respect to the wall surface of the flue gas duct 13, and accordingly, it is possible to prevent the droplet sprayed from the spray nozzle 15 from colliding against the wall surface of the flue gas duct 13.

1×sin α<x  (1)

Note, however, that 1 represents a moved distance over which a droplet of the NH₄Cl solution 14 is moved before the evaporation thereof.

Table 2 shows examples of a shortest distance x from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15 when a jet angle α of a droplet of the NH₄Cl solution 14 sprayed from the nozzle hole 15 a of the spray nozzle 15 with respect to the wall surface of the flue gas duct 13 is about 10° with respect to a gas flow direction of the flue gas 12.

TABLE 2 DROPLET SHORTEST DISTANCE FROM INNER WALL DIAMETER OF FLUE GAS DUCT TO NOZZLE (μm) HOLE OF SPRAY NOZZLE x(m) 40  0.13 (=0.76 × sin 10°) 60 0.28 (=1.6 × sin 10°) 80 0.47 (=2.7 × sin 10°)

As shown in Table 2, when the droplet diameter of the NH₄Cl solution 14 is about 40 μm, the shortest distance x from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15 is about 0.13 m. Also, when the droplet diameter of the NH₄Cl solution 14 is about 60 μm, the shortest distance x from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15 is about 0.28 m. Also, when the droplet diameter of the NH₄Cl solution 14 is about 80 μm, the shortest distance x from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15 is about 0.47 m.

Therefore, when the jet angle α of a droplet of the NH₄Cl solution 14 sprayed from the nozzle hole 15 a of the spray nozzle 15 with respect to the wall surface of the flue gas duct 13 is about 10° with respect to a gas flow direction of the flue gas 12, the nozzle hole 15 a of the spray nozzle 15 needs to be provided 0.13 m or longer away from the flue gas duct 13 when the droplet diameter of the NH₄Cl solution 14 is about 40 μm. Also, when the droplet diameter of the NH₄Cl solution 14 is about 60 μm, the nozzle hole 15 a of the spray nozzle 15 needs to be provided 0.28 m or longer away from the flue gas duct 13. Also, when the droplet diameter of the NH₄Cl solution 14 is about 80 μm, the nozzle hole 15 a of the spray nozzle 15 needs to be provided 0.47 m or longer away from the flue gas duct 13.

Therefore, the nozzle hole 15 a of the spray nozzle 15 is provided at a position 0.5 m or longer away from the wall surface of the flue gas duct 13, for example. As a result, the nozzle hole 15 a of the spray nozzle 15 can be positioned with a sufficient distance for the droplet of the sprayed NH₄Cl solution 14 to be gasified before reaching to the inner wall 13 a of the flue gas duct 13 from the spray nozzle 15 depending on the gas flow velocity of the flue gas 12, the initial velocity of a droplet of the NH₄Cl solution 14 sprayed from the nozzle hole 15 a of the spray nozzle 15, the droplet diameter, the jet angle of the NH₄Cl solution 14 sprayed from the spray nozzle 15 with respect to the flue gas duct 13, the flue gas temperature of the flue gas 12, the droplet temperature of the NH₄Cl solution 14, and the like. As a result, the spray nozzle 15 can supply the NH₄Cl solution 14 into the flue gas duct 13 while preventing the NH₄Cl solution 14 from being adhered to the inner wall 13 a of the flue gas duct 13 through which the flue gas 12 is flowing.

As described above, the spray nozzle 15 is configured as a two-fluid nozzle for simultaneously jetting the NH₄Cl solution 14 and the air 26 for compression. The air 26 is fed from an air supply unit 31 to the spray nozzle 15 via the air supply pipe 27 and then used as compression air when spraying the NH₄Cl solution 14 from the spray nozzle 15. The air 26 thereby makes it possible to spray the NH₄Cl solution 14 jetted from the spray nozzle 15 in a form of fine droplets in the flue gas duct 13.

Moreover, a flow rate of the air 26 supplied from the air supply pipe 27 is adjusted by a valve V2. The size of a droplet of the NH₄Cl solution 14 sprayed from the spray nozzle 15 can be adjusted by the flow rate of the air 26 supplied from the air supply pipe 27.

Moreover, the flow rate of the air 26 jetted from the spray nozzle 15 is preferably in a range of 100 or higher and 10000 or lower in air-water ratio (volume ratio), for example. This is to allow the NH₄Cl solution 14 jetted from the spray nozzle 15 to be sprayed in a form of fine droplets within the flue gas duct 13.

The NH₄Cl solution supply means 16 sprays the NH₄Cl solution 14 into the flue gas duct 13 from the spray nozzle 15 using the spray nozzle 15. However, the present invention is not limited thereto. It is only necessary for the NH₄Cl solution supply means 16 to be able to stably spray the NH₄Cl solution 14 into the flue gas duct 13. FIG. 3 is a diagram showing an example of the configuration of the NH₄Cl solution supply means. As shown in FIG. 3, in the NH₄Cl solution supply means 16, the spray nozzle 15 has a double-pipe structure in which an inner pipe thereof is the NH₄Cl solution supply pipe 25 and an outer pipe thereof is the air supply pipe 27, and the nozzle hole 15 a is provided at the tip portion of the nozzle hole 15 a. The spray nozzle 15 is configured so that the air supply pipe 27 surrounds the NH₄Cl solution supply pipe 25 and the spray nozzle 15 is inserted into the flue gas duct 13. Accordingly, the air 26 flows between the NH₄Cl solution supply pipe 25 and the air supply pipe 27. As a result, it is possible to prevent the heat of the flue gas 12 in the flue gas duct 13 from transferring to the NH₄Cl solution 14 by the air 26. Thus, the NH₄Cl solution 14 can be prevented from being heated by the heat of the flue gas 12, thereby being able to maintain a liquid state thereof up to a point immediately before the NH₄Cl solution 14 is jetted.

Moreover, the NH₄Cl solution supply means 16 includes, within the flue gas duct 13, a blowing pipe 32 inserted into the flue gas duct 13 so as to surround the spray nozzle 15, and an air supply pipe 34 for supplying air 33 into the blowing pipe 32. Moreover, the nozzle hole 15 a of the spray nozzle 15 is provided at a jet hole 35 on a side wall surface of the blowing pipe 32.

The air 33 is used for further dispersing droplets of the NH₄Cl solution 14 within the flue gas duct 13. The air 33 is fed from an air supply unit 36 to the blowing pipe 32 via the air supply pipe 34, and jetted from a gap 37 between the jet hole 35 of the blowing pipe 32 and the nozzle hole 15 a of the spray nozzle 15. By jetting the air 33 from the gap 37, droplets of the NH₄Cl solution 14 sprayed from the spray nozzle 15 can be dispersed within the flue gas duct 13. Moreover, as shown in FIG. 1, the flow rate of the air 33 supplied from the air supply unit 36 is adjusted by a valve V3.

The air 33 is used for preventing NH₄Cl of the NH₄Cl solution 14 sprayed from the spray nozzle 15 from being adhered to the blowing pipe 32 and for suppressing a temperature increase in the spray nozzle 15 so as to prevent the boil of the NH₄Cl solution 14 and the precipitation of ammonium chloride particles. As shown in FIG. 3, since the blowing pipe 32 is inserted into the flue gas duct 13 so as to surround the spray nozzle 15 and the air 33 flows between the blowing pipe 32 and the NH₄Cl solution supply pipe 25 of the spray nozzle 15, the air 33 serves as air for cooling the NH₄Cl solution 14. Therefore, it is possible to prevent the heat of the flue gas 12 in the flue gas duct 13 from transferring to the inside of the NH₄Cl solution supply pipe 25 of the spray nozzle 15 from the outside of the blowing pipe 32. Thus, a temperature increase inside the spray nozzle 15 can be prevented and the heating of the NH₄Cl solution 14 can be prevented, whereby the NH₄Cl solution 14 can be prevented from boiling within the spray nozzle 15. Thus, the NH₄Cl solution 14 can maintain a liquid state thereof up to a point immediately before being jetted. Moreover, corrosion of the spray nozzle 15 can also be prevented.

Metal materials can be used as materials for forming the NH₄Cl solution supply pipe 25 and the air supply pipe 27 since they can prevent a temperature increase inside the spray nozzle 15. For example, as materials for forming the NH₄Cl solution supply pipe 25 and the air supply pipe 27, examples for the NH₄Cl solution supply pipe 25 include a corrosion-resistant metal, for example, a nickel-based heat-resistant and corrosion-resistant alloy such as hastelloy C, and a resin lining steel pipe (low temperature part). Examples for the air supply pipe 27 include carbon steel, stainless steel, and the like.

Moreover, although the spray nozzle 15 employs a two-fluid nozzle for spraying the NH₄Cl solution 14, the present invention is not limited thereto. A single-fluid nozzle typically used for spraying a liquid may be used.

Moreover, since the spray nozzle 15 has a double-pipe structure with the NH₄Cl solution supply pipe 25 and the air supply pipe 27 so that the periphery of the NH₄Cl solution supply pipe 25 is surrounded by the air supply pipe 27, it is possible to prevent the NH₄Cl solution 14 from being heated by the heat of the flue gas 12. However, since the blowing pipe 32 is provided around the spray nozzle 15, it becomes possible to prevent the NH₄Cl solution 14 from being heated by the heat of the flue gas 12 in a more stable manner.

Although the NH₄Cl solution supply pipe 25 is provided inside the air supply pipe 27, the NH₄Cl solution supply pipe 25 may be provided not inside the air supply pipe 27 but outside the air supply pipe 27.

Moreover, the air 26 is supplied from the air supply unit 31 and the air 33 is supplied from the air supply unit 36, i.e., air is supplied respectively from different supply sources. However, the present invention is not limited thereto, and air may be supplied from the same supply source. That is, the air 33 may be air supplied from the air supply unit 31. Moreover, the air 26 may be air supplied from the air supply unit 36.

Moreover, the droplets of the NH₄Cl solution 14, which are sprayed into the flue gas duct 13 from the spray nozzle 15, evaporate by a high ambient temperature of the flue gas 12, thereby producing fine solid particles of NH₄Cl. The fine solid particles of NH₄Cl are broken down into HCl and NH₃ as in the following expression (2) and sublimed. Thus, the NH₄Cl solution 14 is sprayed from the spray nozzle 15, whereby HCl and NH₃ can be produced from the droplets of the sprayed NH₄Cl solution 14. Then, the NH₃ gas and the HCl gas can be therefore supplied into the flue gas duct 13.

NH₄Cl→NH₃+HCl  (2)

Moreover, the temperature of the flue gas 12 in the flue gas duct 13 is high, for example, 320° C. or higher and 420° C. or lower. The NH₄Cl solution supply pipe 25 of the spray nozzle 15 is provided inside the blowing pipe 32, and the air 33 is used for cooling the NH₄Cl solution 14. Thus, by maintaining the liquid state of the NH₄Cl solution 14 up to a point immediately before it is jetted from the spray nozzle 15 and spraying the NH₄Cl solution 14 in a droplet form from the spray nozzle 15, the sprayed droplets of the NH₄Cl solution 14 can be gasified by the high ambient temperature of the flue gas 12.

Moreover, droplets of the NH₄Cl solution 14 sprayed from the spray nozzle 15 are preferably fine droplets having a diameter in a range of 1 nm or larger and 100 μm or smaller on average. Since fine droplets in a range of 1 nm or larger and 100 μm or smaller on average can be generated, the solid particles of NH₄Cl produced from the droplets of the sprayed NH₄Cl solution 14 can be broken down into NH₃ and HCl with a short residence time in the flue gas 12 and can be sublimed. Thus, there is no need to heat the NH₄Cl solution 14 in advance, thereby being able to prevent the degradation and corrosion of the flue gas duct 13 and the spray nozzle 15.

The NH₄Cl solution 14 can be produced by dissolving ammonium chloride (NH₄Cl) powders in water. Since the respective supply amounts of the NH₄Cl powders and the water can be adjusted, the NH₄Cl solution 14 can be adjusted to a predetermined concentration. The NH₄Cl solution 14 may be produced by mixing an HCl solution with an NH₃ solution at a predetermined concentration ratio.

Moreover, the concentration of the NH₄Cl solution 14 is preferably in a range of 20 wt % or higher and 30 wt % or lower when the temperature of a droplet thereof is 20° C., for example. Table 3 shows relationships among a temperature of a droplet of the NH₄Cl solution 14, solubility thereof, and a concentration thereof. This is because solubility of the NH₄Cl solution 14 is approximately determined by the temperature of a droplet thereof as shown in Table 3.

TABLE 3 TEMPERATURE SOLUBILITY CONCENTRATION (° C.) (g/100 g WATER) (wt %) 0 29.4 22.7 20 37.2 27.1 40 45.8 31.4 60 55.2 35.6 80 65.6 39.6 100 77.3 43.6

Although depending on the combustion conditions of the boiler 11, the temperature of the flue gas 12 in the flue gas duct 13 is preferably in a range of 320° C. or higher and 420° C. or lower, more preferably in a range of 320° C. or higher and 380° C. or lower, and further preferably in a range of 350° C. or higher and 380° C. or lower, for example. This is because these temperature ranges make it possible to efficiently generate an NOx removal reaction and an Hg oxidation reaction simultaneously on the denitration catalyst.

Thus, since the NH₄Cl solution 14 in a liquid state is supplied into the flue gas duct 13 from the spray nozzle 15 while preventing the NH₄Cl solution 14 from being adhered to the inner wall 13 a of the flue gas duct 13, the HCl gas and the NH₃ gas obtained by decomposing the NH₄Cl solution 14 by the high ambient temperature of the flue gas 12 can be evenly supplied in the flue gas duct 13 without concentration unevenness. As a result, the concentration distributions of the HCl gas and the NH₃ gas in the flue gas 12 can be made uniform. Moreover, since the NH₄Cl solution 14 can be prevented from being adhered to the wall surface of the flue gas duct 13 before the gasification thereof, it is possible to prevent a breakage of the flue gas duct 13 caused by corrosion of the flue gas duct 13, or the like.

Moreover, as shown in FIG. 1, the HCl gas and the NH₃ gas generated from the droplets of the NH₄Cl solution 14 are fed to the reduction-denitration device 18 together with the flue gas 12. FIG. 4 is a diagram specifically showing the configuration of the denitration device. As shown in FIG. 4, the reduction-denitration device 18 is formed by three denitration catalyst layers 38-1 to 38-3. Moreover, the gas flow of the flue gas 12 is equalized by a current plate 39 before the flue gas 12 passes through the reduction-denitration device 18. The NH₃ gas generated by the decomposition of NH₄Cl is used in the reduction-denitration device 18 for the NOx reduction and denitration, and the HCl gas is used for the Hg oxidation. Thus, the amounts of NOx and Hg are reduced in the flue gas 12.

That is, NH₃ performs the reduction and denitration of NOx as in the following expression (3) on the denitration catalysts of the denitration catalyst layers 38-1 to 38-3 filled in the reduction-denitration device 18, and HCl performs the mercury oxidation of Hg as in the following expression (4).

4NO+4NH₃+O₂→4N₂+6H₂O  (3)

Hg+1/2O₂+2HCl→HgCl₂+H₂O  (4)

Moreover, although the reduction-denitration device 18 is composed of the three denitration catalyst layers 38-1 to 38-3, the present invention is not limited thereto. The reduction-denitration device 18 can suitably change the number of denitration catalyst layers depending on the denitration ability thereof.

Moreover, as shown in FIG. 1, after NOx and Hg in the flue gas 12 are respectively subjected to reduction and oxidation in the reduction-denitration device 18, the flue gas 12 is passed through the air heater 19 and the precipitator 20, and fed to the wet desulfurization device 22. Moreover, a heat recovery unit may be provided between the air heater 19 and the precipitator 20.

In the wet desulfurization device 22, the flue gas 12 is fed from a wall surface side at a bottom portion in a device body 41, and the limestone-gypsum slurry 21 used as an alkali absorbent is supplied into the device body 41 by an absorbent feed line 42 so as to be jetted toward a top portion side from a nozzle 43. The flue gas 12 rising from the bottom portion side in the device body 41 and the falling limestone-gypsum slurry 21 after being jetted from the nozzle 43 are made opposed to each other to achieve gas-liquid contact. HgCl and sulfur oxide (SOx) in the flue gas 12 are absorbed into the limestone-gypsum slurry 21, thereby being separated and reduced in amount in the flue gas 12. As a result, the flue gas 12 is purged. The flue gas 12 purged by the limestone-gypsum slurry 21 is discharged as purged gas 44 from the top portion side and then discharged to the outside of the system from a stack 45.

The limestone-gypsum slurry 21 used for the desulfurization of the flue gas 12 is produced by mixing a limestone slurry CaCO₃ obtained by dissolving limestone powders into water, a gypsum slurry CaSO₄ obtained by reacting limestone with SOx in the flue gas 12 and further oxidizing the resultant, and water. For example, the limestone-gypsum slurry 21 may be the one obtained by pumping up a liquid accumulated in a bottom portion 55 of the device body 41 in the wet desulfurization device 22. In the device body 41, SOx in the flue gas 12 reacts with the limestone-gypsum slurry 21 as in the following expression (5).

CaCO₃+SO₂+0.5H₂O→CaSO₃.0.5H₂O+CO₂  (5)

On the other hand, the limestone-gypsum slurry 21, which has absorbed SOx in the flue gas 12, is mixed with water 46 supplied into the device body 41, and is subjected to an oxidation treatment by air 47 supplied to the bottom portion 55 of the device body 41. Here, the limestone-gypsum slurry 21 flowed down in the device body 41 reacts with the water 46 and the air 47 as in the following expression (6).

CaSO₃.0.5H₂O+0.5O₂+1.5H₂O→CaSO₄.2H₂O  (6)

Moreover, the limestone-gypsum slurry 21, which is accumulated in the bottom portion 55 of the wet desulfurization device 22 and has been used for desulfurization, is extracted from the bottom portion 55 after the oxidation treatment and fed to a dehydrator 48. Thereafter, it is discharged to the outside of the system as a dehydrated cake (gypsum) 49 containing mercury chloride (HgCl). As the dehydrator 48, a belt filter or the like may be used, for example. Moreover, filtrate which has been dehydrated (dehydrated filtrate) is subjected to an effluent treatment such as the removal of suspended solids and a heavy metal in the dehydrated filtrate, and pH adjustment of the dehydrated filtrate, for example. Part of the dehydrated filtrate which has been subjected to the effluent treatment is sent back to the wet desulfurization device 22, and another part of the dehydrated filtrate is treated as discharged water.

Moreover, although the limestone-gypsum slurry 21 is used as an alkali absorbent, another solution can be used as an alkali absorbent as long as it can absorb HgCl in the flue gas 12.

The limestone-gypsum slurry 21 is not limited to be supplied by a method in which the limestone-gypsum slurry 21 is jetted toward the top portion side from the nozzle 43, and it may be flowed down from the nozzle 43 so as to be opposed to the flue gas 12, for example.

<Control for Sprayed Amount of NH₄Cl Solution>

A flowmeter 51 for measuring a flow rate of the flue gas 12 is provided on the upstream side of the spray nozzle 15. The flow rate of the flue gas 12 is measured by the flowmeter 51. The value of the flow rate of the flue gas 12 measured by the flowmeter 51 is sent to a control device 52. Based on the value of the flow rate of the flue gas 12, the flow rate, angle, initial velocity, and the like of the NH₄Cl solution 14 jetted from the spray nozzle 15 can be adjusted.

Moreover, an NOx concentration meter 53 is provided at an outlet side of the wet desulfurization device 22. The value of the NOx concentration in the purged gas 44 measured by the NOx concentration meter 53 is transmitted to the control device 52. The control device 52 can check the NOx reduction ratio in the reduction-denitration device 18 based on the value of the NOx concentration in the purged gas 44 measured by the NOx concentration meter 53. Thus, the NH₄Cl concentration, the supply flow rate, and the like of the NH₄Cl solution 14 are controlled based on the value of the NOx concentration in the purged gas 44 measured by the NOx concentration meter 53, whereby the NH₄Cl concentration of the NH₄Cl solution 14 sprayed from the spray nozzle 15 can be made to satisfy a predetermined denitration ability.

Moreover, mercury (Hg) concentration meters 54-1 and 54-2 for measuring an Hg content in the flue gas 12 discharged from the boiler 11 are provided in the flue gas duct 13. The Hg concentration meter 54-1 is provided in the flue gas duct 13 between the boiler 11 and the spray nozzle 15, and the Hg concentration meter 54-2 is provided between the reduction-denitration device 18 and the heat exchanger 19. The values of the Hg concentrations in the flue gas 12 measured by the Hg concentration meters 54-1 and 54-2 are transmitted to the control device 52. The control device 52 can check Hg contents contained in the flue gas 12 based on the values of the Hg concentrations in the flue gas 12 measured by the Hg concentration meters 54-1 and 54-2. Since the NH₄Cl concentration and the supply flow rate of the NH₄Cl solution 14 are controlled based on the values of the Hg concentrations in the flue gas 12 measured by the Hg concentration meters 54-1 and 54-2, the NH₄Cl concentration and the supply flow rate of the NH₄Cl solution 14 sprayed from the spray nozzle 15 can be made to satisfy the predetermined denitration ability and to maintain the Hg oxidation ability.

Moreover, an oxidation-reduction potential measurement control device (ORP controller) 56 for measuring an oxidation-reduction potential of the limestone-gypsum slurry 21 is provided at the bottom portion 55 of the wet desulfurization device 22. The value of the oxidation-reduction potential of the limestone-gypsum slurry 21 is measured by the ORP controller 56. Based on the measured oxidation-reduction potential value, the supply amount of the air 47 supplied to the bottom portion 55 of the wet desulfurization device 22 is adjusted. The supply amount of the air 47 supplied to the bottom portion 55 is adjusted, so that it is possible to prevent the oxidized Hg trapped in the limestone-gypsum slurry 21 accumulated in the bottom portion 55 of the wet desulfurization device 22 from being reduced and also from being emitted from the stack 45.

The oxidation-reduction potential of the limestone-gypsum slurry 21 in the wet desulfurization device 22 is preferably in a range of 150 mV or higher and 600 mV or lower, for example, in order to prevent the re-scattering of Hg from the limestone-gypsum slurry 21. This is because if the oxidation-reduction potential is within the above-described range, Hg trapped in the limestone-gypsum slurry 21 as HgCl₂ is in a stable region, and it is therefore possible to prevent the re-scattering thereof into the air.

Moreover, although NH₄Cl is used as a reduction-oxidation auxiliary agent in the air pollution control device 10 according to the present embodiment, ammonium halide other than NH₄Cl, such as ammonium bromide (NH₄Br) or ammonium iodide (NH₄I), may be used as a reduction-oxidation auxiliary agent and a solution obtained by dissolving such ammonium halide in water may be used.

As described above, according to the air pollution control device 10 of the present embodiment, since the NH₄Cl solution supply means 16 supplies the NH₄Cl solution 14 from the spray nozzle 15 while preventing the NH₄Cl solution 14 from being adhered to the inner wall 13 a of the flue gas duct 13 through which the flue gas 12 flows, HCl and NH₃, which are produced upon the gasification of the droplets of the NH₄Cl solution 14 sprayed into the flue gas duct 13, can be mixed with the flue gas 12. Thus, HCl and NH₃ can be evenly supplied in a stable manner into the flue gas duct 13 with no concentration unevenness. As a result, in the reduction-denitration device 18, it is possible to improve the Hg oxidation ability and the NOx reducing ability. Moreover, since it is possible to prevent the NH₄Cl solution 14 from being adhered to the wall surface of the flue gas duct 13 before the gasification thereof, it is also possible to prevent a breakage of the flue gas duct 13 or the like resulting from corrosion of the flue gas duct 13 or the like from occurring.

Second Embodiment

An air pollution control device according to the second embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the second embodiment of the present invention has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention, a description will be made in the present embodiment with reference to only a diagram showing a configuration of jet nozzles in a flue gas duct. FIG. 5 is a diagram showing a cross-section of the flue gas duct of the air pollution control device according to the second embodiment of the present invention as viewed from a flow direction of flue gas. Note that the elements overlapping with the configuration of the air pollution control device according to the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 5, the air pollution control device according to the present embodiment is formed by arranging a plurality of spray nozzles 15 in the flue gas duct 13 so as to satisfy the following expression (7).

a≦b/5  (7)

Note however that a denotes a distance between nozzle holes of the spray nozzle, and b denotes a long side length of the lengths of the cross-section of the flue gas duct.

Since the plurality of spray nozzles 15 are disposed in the flue gas duct 13 so as to satisfy the above expression (7), the number of spray nozzles 15 to be disposed in the flue gas duct 13 can be increased as compared to the conventional technique while supplying the NH₄Cl solution 14 into the flue gas duct 13 from the spray nozzles 15 in such a way to prevent the adhesion of the NH₄Cl solution 14 to the inner wall 13 a of the flue gas duct 13. Thus, the spray nozzles 15 can be appropriately disposed in the flue gas duct 13. Therefore, the amounts of NH₃ and HCl sprayed into the flue gas 12 can be further increased, thereby promoting the mixing of the NH₃ gas and the HCl gas into the flue gas 12.

Moreover, the plurality of spray nozzles 15 are preferably disposed in the flue gas duct 13 so as to satisfy the following expression (8).

a≦b/10  (8)

Thus, according to the air pollution control device of the present embodiment, since the plurality of spray nozzles 15 are disposed in the flue gas duct 13 so as to satisfy the above expression (7), it is possible to increase the supply amounts of the NH₃ gas and the HCl gas into the flue gas 12 and to reduce concentration unevenness of the HCl gas and the NH₃ gas in the flue gas duct 13. Thus, it is possible to further improve the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Third Embodiment

An air pollution control device according to the third embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the third embodiment of the present invention has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention, a description will be made in the present embodiment with reference to only diagrams showing the configuration of jet nozzles in a flue gas duct. FIG. 6 is a diagram showing a cross-section of the flue gas duct of the air pollution control device according to the third embodiment of the present invention as viewed from a flow direction of flue gas. FIG. 7 is a diagram showing the configuration of the spray nozzle in a simplified manner. Note that the elements overlapping with the configurations of the air pollution control devices according to the first and second embodiments will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIGS. 6 and 7, the air pollution control device according to the present embodiment includes spray nozzles 61 each having four nozzle holes 15 a for spraying the NH₄Cl solution 14. By increasing the number of the nozzle holes 15 a of the spray nozzle 61, it is possible to increase an amount of the NH₄Cl solution 14 sprayed into the flue gas duct 13 from one spray nozzle 61, thereby further promoting the mixing of the HCl gas and the NH₃ gas with the flue gas 12. As a result, it is possible to further improve the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Moreover, a distance c between the nozzle holes 15 a is preferably 0.3 m or shorter.

Given that a droplet is about 40 μm, the moved distance of the droplet in the horizontal direction (i.e., the shortest distance x from the inner wall 13 a of the flue gas duct 13 to the nozzle hole 15 a of the spray nozzle 15) is 0.13 m from Table 2. Since the droplet, after being evaporated and sublimed, flows in the gas flow direction, it is desirable for the homogenization of the concentration distribution that the droplets sprayed from the two nozzle holes 15 a overlap with each other before the evaporation and sublimation thereof. A partial enlarged view of the spray nozzle 61 is shown in FIG. 8. As shown in FIG. 8, the sum of distances over which the droplets emitted from the two nozzle holes 15 a move in the horizontal direction before the evaporation thereof is 0.26 m (=0.13×2). Thus, by setting the distance c between the nozzle holes 15 a to 0.3 m or shorter, droplets jetted from the two nozzle holes 15 a can be overlapped with each other. Moreover, it is often the case where a droplet diameter realistically used is in a range of 40 μm or larger and 80 μm or smaller in view of the controllability and device dimension thereof. When the droplet diameter is about 40 μm as the lower limit thereof, by setting the distance c between the nozzle holes 15 a to 0.3 m or shorter, droplets sprayed from the two nozzle holes 15 a can be overlapped with each other.

Moreover, although each spray nozzle 61 is provided with four nozzle holes 15 a in the air pollution control device according to the present embodiment, the present invention is not limited thereto. Two, three, or five or more nozzle holes 15 a may be provided.

Fourth Embodiment

An air pollution control device according to the fourth embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the fourth embodiment of the present invention has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention, a description will be made in the present embodiment with reference to only a diagram showing the configuration of jet nozzles in a flue gas duct. The air pollution control device according to the fourth embodiment of the present invention can change a sprayed amount from each of spray nozzles 15-11 to 15-24 when twenty-four spray nozzles 15 are provided in the flue gas duct 13. FIG. 9 is a diagram showing a cross-section of the flue gas duct of the air pollution control device according to the fourth embodiment of the present invention as viewed from a flow direction of flue gas. Note that the elements overlapping with the configurations of the air pollution control devices according to the first to third embodiments will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 9, in the air pollution control device according to the present embodiment, a sprayed amount of each of spray nozzles 15-1, 15-11 to 15-13, 15-23, and 15-24 provided on the short side of the flue gas duct 13 is made greater than a sprayed amount of each of spray nozzles 15-2 to 15-10 and 15-14 to 15-22 provided on the long side of the flue gas duct 13. For example, if the sprayed amount of each of the spray nozzles 15-2 to 15-10 and 15-14 to 15-22 is assumed to be 1, the sprayed amount of each of the spray nozzles 15-1, 15-11 to 15-13, 15-23, and 15-24 is set to 1.5.

Since the sprayed amount of the NH₄Cl solution 14 sprayed from each of the spray nozzles 15-1, 15-11 to 15-13, 15-23, and 15-24 provided on the short side of the flue gas duct 13 is made greater than the sprayed amount of the NH₄Cl solution 14 sprayed from each of the spray nozzles 15-2 to 15-10 and 15-14 to 15-22 provided on the long side of the flue gas duct 13, it is possible to efficiently spray the NH₄Cl solution 14 up to end portions of the flue gas duct 13. Thus, the HCl gas and the NH₃ gas can be supplied also to the flue gas 12 flowing near the end portions of the flue gas duct 13, thereby further improving the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Moreover, in the air pollution control device according to the present embodiment, when the sprayed amount of each of the spray nozzles 15-2 to 15-10 and 15-14 to 15-22 provided on the long side of the flue gas duct 13 is assumed to be 1, the sprayed amount of each of the spray nozzles 15-1, 15-11 to 15-13, 15-23, and 15-24 provided on the short side of the flue gas duct 13 is set to 1.5. However, the present invention is not limited thereto. A ratio between the spray nozzles provided on the short side of the flue gas duct 13 and the spray nozzles provided on the long side of the flue gas duct 13 is suitably adjusted depending on the NOx concentration and the Hg concentration in the flue gas 12, the sprayed amount of the NH₄Cl solution 14, and the like.

Although twenty-four spray nozzles 15-11 to 15-24 are provided within the flue gas duct 13 in the air pollution control device according to the present embodiment, the present invention is not limited thereto. A plurality of spray nozzles may be provided depending on an installation area in the flue gas duct 13, and the like.

Fifth Embodiment

An air pollution control device according to the fifth embodiment of the present invention will be described with reference to the drawings. FIG. 10 is a conceptual diagram showing the configuration of the air pollution control device according to the fifth embodiment of the present invention in a simplified manner. Since the air pollution control device according to the present embodiment has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment, identical elements will be denoted by the same reference numerals and the redundant description will be omitted.

As shown in FIG. 10, an air pollution control device 70 according to the present embodiment includes a mixer (mixing means) 71, which is provided on the downstream side of the region of the flue gas duct 13 of the air pollution control device 10 shown in FIG. 1 according to the first embodiment at which NH₄Cl is gasified, for promoting the mixing of a hydrogen chloride (HCl) gas as an oxidizing gas and an ammonia (NH₃) gas as a reducing gas, which are produced when NH₄Cl is gasified, with the flue gas 12.

The flue gas 12 is fed to the mixer 71 after containing therein the HCl gas and the NH₃ gas produced from the droplets of the NH₄Cl solution 14 sprayed by the NH₄Cl solution supply means 16. The flue gas 12 is stirred by the mixer 71, and accordingly, mixing of the HCl gas and the NH₃ gas with the flue gas 12 can be promoted, thereby obtaining uniform concentration distributions of the HCl gas and the NH₃ gas in the flue gas 12.

The mixer 71 of the present embodiment is provided on the downstream side of the region where the NH₄Cl solution 14 sprayed from the spray nozzle 15 is gasified. Under typical plant operating conditions, it is preferable that the mixer 71 is provided at 1 m or longer downstream side of the supply position at which the NH₄Cl solution 14 is supplied. This is because under realistic plant operating conditions, if the mixer 71 is at a position less than 1 m away from the supply position at which the NH₄Cl solution 14 is supplied, the droplets of the NH₄Cl solution 14 often contact with the mixer 71 before the gasification thereof. Thus, the mixer 71 is provided at 1 m or more downstream side of the supply position at which the NH₄Cl solution 14 is supplied, whereby the mixing of the HCl gas and the NH₃ gas in the flue gas 12 can be further promoted. Moreover, in view of a realistic device layout, the mixer 71 is away from the supply position of the NH₄Cl solution 14 with a distance up to about 10 m.

Moreover, a configuration of the mixer 71 is shown in FIGS. 11 to 14. FIG. 11 is a plan view showing an example of the mixer. FIG. 12 is a plan view of a swirling flow inductive member forming the mixer. FIG. 13 is a front view of the swirling flow inductive member, and FIG. 14 is a perspective view of the swirling flow inductive member. Note that in FIGS. 11 to 14, the part of reference numeral 73 is shown with hatching in order to clarify a difference with the member of reference numeral 74.

As shown in FIG. 11, the mixer 71 of the present embodiment is formed by a unit of six swirling flow inductive members 72 for generating a swirling flow in the flue gas 12, which are disposed so as to be perpendicular to the flow direction of the flue gas 12. As shown in FIGS. 12 to 14, the swirling flow inductive member 72 includes a pair of first swirling flow inductive plates 73 having opposed surfaces 73 a on an inlet side of the flue gas 12, and a pair of second swirling flow inductive plates 74 having opposed surfaces 74 a on a discharge side of the flue gas 12. At a flat plate-shaped intermediate member 75 as a connecting portion for connecting the first swirling flow inductive plates 73 and the second swirling flow inductive plates 74, the first swirling flow inductive plate 73 and the second swirling flow inductive plate 74 are connected thereto in such a way that the opposed surface 73 a of the first swirling flow inductive plate 73 and the opposed surface 74 a of the second swirling flow inductive plate 74 are directed differently. In the present embodiment, the opposed surface 73 a of the first swirling flow inductive plate 73 and the opposed surface 74 a of the second swirling flow inductive plate 74 are disposed so as to be different from each other by about 90°.

The first swirling flow inductive plate 73 and the second swirling flow inductive plate 74 are each formed in an approximately triangular shape. Moreover, since the first swirling flow inductive plates 73 are provided on the inlet side of the flue gas 12 and the second swirling flow inductive plates 74 are provided on the discharge side of the flue gas 12, the first swirling flow inductive plates 73 are positioned below the second swirling flow inductive plates 74 when the swirling flow inductive member 72 is viewed from the front thereof. Moreover, the intermediate member 75 is a flat plate and functions as a pivot for connecting the first swirling flow inductive plates 73 and the second swirling flow inductive plates 74. Moreover, the first swirling flow inductive plate 73 is provided with a lower support plate 76, and the second swirling flow inductive plate 74 is provided with an upper support plate 77. The swirling flow inductive members 72 adjacent to each other are connected with each other by the lower support plates 76 and the upper support plates 77 thereof.

As shown in FIG. 14, when the flue gas 12 flows into the swirling flow inductive member 72, the flue gas 12 collides against the reverse sides of the opposed surfaces 73 a of the first swirling flow inductive plates 73. As a result, the gas flow thereof is changed, and the flue gas 12 thereby flows in a direction of the second swirling flow inductive plates 74. Thereafter, the flue gas 12 collides against the reverse sides of the opposed surfaces 74 a of the second swirling flow inductive plates 74, thereby further changing the gas flow thereof. Thus, the flue gas 12 changes the gas flow thereof by the first swirling flow inductive plates 73 and the second swirling flow inductive plates 74, thereby flowing in such a way as to detour the first swirling flow inductive plates 73 and the second swirling flow inductive plates 74 and flowing while revolving from the inflow direction of the flue gas 12 in the swirling flow inductive member 72 toward the discharging direction of the flue gas 12.

Moreover, although the opposed surfaces 73 a of the first swirling flow inductive plate 73 and the opposed surfaces 74 a of the second swirling flow inductive plate 74 are disposed so as to be directed differently by about 90° in the present embodiment, the present invention is not limited thereto. The angle between the direction of the opposed surface 73 a of the first swirling flow inductive plate 73 and the direction of the opposed surface 74 a of the second swirling flow inductive plate 74 may be any angle as long as they can flow the flue gas 12 flowed into the swirling flow inductive member 72 while making it revolve from the inflow direction of the flue gas 12 in the swirling flow inductive member 72 toward the discharging direction of the flue gas 12.

Moreover, although the mixer 71 is formed as a unit in which six swirling flow inductive members 72 are disposed perpendicularly to the flow direction of the flue gas 12 as shown in FIG. 11 in the present embodiment, the present invention is not limited thereto. The number of swirling flow inductive members 72 to be disposed is appropriately changed depending on the area of the flue gas duct 13, and the like.

Moreover, although the mixer 71 is formed as a single unit in which six swirling flow inductive members 72 are disposed in the flow direction of the flue gas 12 in the present embodiment, the present invention is not limited thereto. A plurality of units each having a plurality of swirling flow inductive members 72 disposed in the flow direction of the flue gas 12 may be provided in multiple stages. Moreover, the mixer 71 of the present embodiment may be provided with a unit in which a plurality of swirling flow inductive members 72 are disposed in a direction perpendicular to the flow direction of the flue gas 12, and also a plurality of units each having a plurality of swirling flow inductive members 72 disposed in the flow direction of the flue gas 12 may be provided.

FIG. 15 is a diagram schematically showing a gas flow of the flue gas when the mixer is installed within the flue gas duct, and FIG. 16 is a partial enlarged view of FIG. 15. Note that in FIGS. 15 and 16, six swirling flow inductive members 72 are provided in a width direction of the flue gas duct 13 as in FIG. 11. As shown in FIGS. 15 and 16, when the flue gas 12 passes through the swirling flow inductive member 72, the flue gas 12 collides against the first swirling flow inductive plates 73 and the second swirling flow inductive plates 74. As a result, the gas flow thereof is changed, and the flue gas 12 thereby flows from the lower side of the flue gas duct 13 toward the upper side thereof in such a way as to detour the first swirling flow inductive plates 42 and the second swirling flow inductive plates 43. Therefore, the flue gas 12 can flow, while revolving, from the lower side of the flue gas duct 13 toward the upper side thereof. As a result, it is possible to promote the mixing of the HCl gas and the NH₃ gas with the flue gas 12.

Moreover, since the mixer 71 is provided on the downstream side of the region where the NH₄Cl solution 14 sprayed from the spray nozzle 15 is gasified, it is possible to prevent droplets of the NH₄Cl solution 14 from contacting with the mixer 71 before the gasification thereof. Thus, it is possible to prevent a breakage of the flue gas duct 13 due to heat shock, corrosion of the flue gas duct 13, deposition of ash in the flue gas 12, and the like from occurring.

FIGS. 17 and 18 are cross-sectional views taken along the line A-A in FIG. 4. FIG. 17 is a diagram schematically showing an example of the NH₃ gas concentration distribution in the flue gas when the mixer is not installed within the flue gas duct. FIG. 18 is a diagram schematically showing an example of the NH₃ gas concentration distribution in the flue gas when the mixer is installed within the flue gas duct. Note that in FIG. 17, reference numeral 104 indicates a flue gas duct in the conventional air pollution control system shown in FIG. 29.

As shown in FIGS. 17 and 18, unevenness in the NH₃ gas concentration distribution in the flue gas 12 immediately before flowing into the reduction-denitration device 18 in a case where the mixer 71 is not installed is greater than that in a case where the mixer 71 is installed within the flue gas duct 13.

Thus, since the mixing of the NH₃ gas with the flue gas 12 in the flue gas duct 13 can be promoted by providing the mixer 71 on the downstream side of the region where the NH₄Cl solution 14 sprayed from the spray nozzle 15 is gasified, unevenness in the NH₃ gas concentration distribution in the flue gas 12 can be suppressed. Unevenness in the NH₃ gas concentration distribution is kept within about 5%, for example, thereby making the distribution substantially uniform. Thus, it is possible to improve an NOx reduction efficiency with the denitration catalyst in the reduction-denitration device 18.

Moreover, since the mixer 71 is provided on the downstream side of the region where the NH₄Cl solution 14 sprayed from the spray nozzle 15 is gasified, the mixing of the HCl gas, in addition to the NH₃ gas, with the flue gas 12 in the flue gas duct 13 can be also promoted. Thus, unevenness in the HCl gas concentration distribution in the flue gas 12 can be suppressed. Unevenness in the HCl gas concentration distribution is also kept within about 5%, for example, thereby making the distribution substantially uniform. Thus, it is possible to improve the Hg oxidation ability with the denitration catalyst in the reduction-denitration device 18.

Moreover, as shown in FIGS. 11 to 14, a width L and a height D of the swirling flow inductive member 72 are preferably within the ranges of the following expressions (9) and (10).

MIN(B,H)/10≦L≦MIN(B,H)  (9)

MIN(B,H)/10≦D≦5×MIN(B,H)  (10)

Note that B represents a long side of the cross-section of the flue gas duct at the installation position, H represents a short side of the cross-section of the flue gas duct, and MIN(B,H) represents the shorter one of the long side B of the cross-section of the flue gas duct and the short side H of the cross-section of the flue gas duct. When the long side B and the short side H of the cross-section of the flue gas duct have the same length, either one may be used.

The reason why the swirling flow inductive member 72 is made to fall within the ranges of the expressions (9) and (10) above is that determination is required in view of the conditions for pressure loss of the mixer 71, unevenness in the NH₃ concentration in the flue gas 12, the workability when manufacturing, the realistic operating conditions, the maintenance performance, and the like, as will be shown below.

FIG. 19 is a diagram showing the relationship between the pressure loss of the mixer and the dimension of the mixer. As shown in FIG. 19, in order for the pressure loss of the mixer to be 25 mmAq or lower, the following expression (11) needs to be satisfied. Moreover, in order for the concentration unevenness in the NH₃ concentration in the flue gas 12 to be kept within 5%, the following expression (12) needs to be satisfied.

MIN(B,H)×D/L ²≦2  (11)

MIN(B,H)×D/L ²≦5  (12)

That is, as pressure loss conditions, in order for the pressure loss of the mixer 71 to be 25 mmAq or lower, the expression (11) above needs to be satisfied. Moreover, in order for the concentration unevenness in the NH₃ concentration in the flue gas 12 to be kept at 5% or lower as an advantageous effect of the mixer 71, the expression (12) above needs to be satisfied.

Moreover, the mixer 71 needs to satisfy the above expression (9) and the following expression (13) in view of the workability when manufacturing, the realistic operating conditions, and the maintenance performance.

MIN(B,H)/10≦L≦MIN(B,H)  (9)

MIN(B,H)/10≦D  (13)

From the above expressions (11) and (12), D can be expressed as in the following expression (14).

2L ²/MIN(B,H)≦D≦5L ²/MIN(B,H)  (14)

By substituting the above expression (9) into the above expression (14), D can be expressed as in the following expression (15).

MIN(B,H)/50≦D≦5×MIN(B,H)  (15)

If the above expression (13) is taken into consideration in the above expression (15), D can be expressed as in the above expression (10).

MIN(B,H)/10≦D≦5×MIN(B,H)  (10)

When the width L and the height D of the swirling flow inductive member 72 are within the ranges of the above expressions (9) and (10) as described above, a plurality of swirling flow inductive members 72 can be installed within the flue gas duct 13. Thus, it is possible to promote the mixing of HCl and NH₃ with the flue gas 12.

Moreover, the shapes of the first swirling flow inductive plate 73 and the second swirling flow inductive plate 74 are not limited to the triangular shapes formed so as to extend from the upper support plate 77 and the lower support plate 76 to the intermediate member 75. Any shape may be employed as long as it can generate a swirling flow in the flue gas 12, thereby promoting the mixing of HCl and NH₃ with the flue gas 12. For example, the shapes of the first swirling flow inductive plate 73 and the second swirling flow inductive plate 74 may be of a curved line type, a corrugated type, or the like, extending from one ends of the second swirling flow inductive plate 74 and the first swirling flow inductive plate 73 toward the other ends thereof.

Thus, according to the air pollution control device 70 of the present embodiment, a plurality of swirling flow inductive members 72 are provided in a cross-sectional direction of the flue gas duct 13 as the mixer 71, and accordingly, the mixing of HCl and NH₃ with the flue gas 12 can be promoted. Thus, it is possible to achieve homogenization of the concentration distributions of NH₃ and HCl generated by the gasification of the NH₄Cl solution 14 sprayed from the spray nozzle 15. As a result, it is possible to improve an Hg oxidation ability and an NOx reducing ability by the denitration catalyst in the reduction-denitration device 18, and it is also possible to prevent a breakage of the flue gas duct 13 due to heat shock, corrosion of the flue gas duct 13, deposition of ash in the flue gas 12, and the like from occurring.

Sixth Embodiment

An air pollution control device according to the sixth embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the sixth embodiment of the present invention has the same configuration as the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention and the air pollution control device 70 shown in FIG. 10 according to the fifth embodiment of the present invention, a description will be made in the present embodiment with reference to only diagrams showing the configuration of a flue gas duct. FIG. 20 is a diagram showing the flue gas duct of the air pollution control device according to the sixth embodiment of the present invention as viewed from a short side direction thereof. FIG. 21 is a diagram showing the flue gas duct as viewed from a long side direction thereof. Note that the elements overlapping with the configurations of the air pollution control devices according to the first to fifth embodiments will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIGS. 20 and 21, in the air pollution control device according to the present embodiment, the flue gas duct 13 includes protruding members 81 provided on the inner wall 13 a of the flue gas duct 13 on the downstream side of the supply position at which the NH₄Cl solution 14 is supplied into the flue gas duct 13 and in the region where droplets of the NH₄Cl solution 14 have been gasified. By providing the protruding members 81 on the inner wall 13 a of the flue gas duct 13, an open width of the flue gas duct 13 through which the flue gas 12 can flow is reduced. Thus, it is possible to generate a vortex due to the gas flow of the flue gas 12 in the vicinity of the wall surface of the flue gas duct 13. As a result, it is possible to promote the mixing of the HCl gas and the NH₃ gas in the flue gas 12 flowing near the wall surface of the flue gas duct 13, thereby improving the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Moreover, the installation position of the protruding member 81 is preferably provided in a region where droplets of the NH₄Cl solution 14 sprayed from the spray nozzle 15 have already been gasified in order to prevent the droplets of the NH₄Cl solution 14 from colliding against the protruding member 81. Particularly, the protruding member 81 is preferably provided at 1 m or more downstream side away from the spray nozzle 15.

Moreover, although the shape of the protruding member 81 is a plate shape in the air pollution control device according to the present embodiment, the present invention is not limited thereto. Other shapes such as a box shape or a triangular shape may be used.

Seventh Embodiment

An air pollution control device according to the seventh embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the seventh embodiment of the present invention has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention and the air pollution control device 70 shown in FIG. 10 according to the fifth embodiment of the present invention, a description will be made in the present embodiment with reference to only diagrams showing a configuration of a flue gas duct. FIG. 22 is a diagram showing the flue gas duct of the air pollution control device according to the seventh embodiment of the present invention as viewed from a short side direction thereof. FIG. 23 is a diagram showing the flue gas duct as viewed from a long side direction thereof. Note that the elements overlapping with the configurations of the air pollution control devices according to the first to sixth embodiments will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIGS. 22 and 23, the air pollution control device according to the present embodiment is formed with a narrowed portion 82 for narrowing a passage in the flue gas duct 13 provided on the downstream side of the supply position at which the NH₄Cl solution 14 is supplied into the flue gas duct 13. Since the narrowed portion 82 for narrowing the passage in the flue gas duct 13 is provided on the wall surface of the flue gas duct 13, it is possible to generate a vortex due to the gas flow of the flue gas 12 in the vicinity of the wall surface of the flue gas duct 13. As a result, it is possible to promote the mixing of the HCl gas and the NH₃ gas in the flue gas 12 flowing near the wall surface of the flue gas duct 13. Thus, it is possible to suppress concentration unevenness of the HCl gas and the NH₃ gas in the flue gas 12, thereby improving the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Moreover, although the passage of the flue gas duct 13 is narrowed to form the narrowed portion 82 in the air pollution control device according to the present embodiment, the present invention is not limited thereto. For example, as shown in FIGS. 24 and 25, a narrowed member 83 having the same shape as the narrowed portion 82 may be provided on the wall surface of the flue gas duct 13. Thus, it is possible to generate a vortex due to the gas flow of the flue gas 12 in the vicinity of the narrowed member 83. As a result, it is possible to promote the mixing of HCl and NH₃ in the flue gas 12 flowing near the wall surface of the flue gas duct 13.

Moreover, as with the case of the protruding member 81 in the sixth embodiment, the installation position of the narrowed portion 82 is preferably provided in the region where the droplets of the NH₄Cl solution 14 sprayed from the spray nozzle 15 have already been gasified in order to prevent the droplets of the NH₄Cl solution 14 from colliding against the narrowed portion 82. Particularly, the narrowed portion 82 is preferably provided at a position 1 m or more downstream side away from the spray nozzle 15.

Eighth Embodiment

An air pollution control device according to the eighth embodiment of the present invention will be described with reference to the drawings. Since the air pollution control device according to the eighth embodiment of the present invention has a similar configuration to the air pollution control device 10 shown in FIG. 1 according to the first embodiment of the present invention and the air pollution control device 70 shown in FIG. 10 according to the fifth embodiment of the present invention, a description will be made in the present embodiment with reference to only diagrams showing a configuration of a flue gas duct. FIG. 26 is a diagram showing part of the air pollution control device according to the eighth embodiment of the present invention. FIG. 27 is a partial enlarged perspective view showing the area of reference symbol Z in FIG. 26. Note that the elements overlapping with the configurations of the air pollution control devices according to the first to seventh embodiments will be denoted by the same reference numerals and the description thereof will be omitted.

As shown in FIGS. 26 and 27, the air pollution control device according to the present embodiment is formed by providing guide vanes 84 provided on the upstream side of the reduction-denitration device 18 with mixing promoting auxiliary members 85 for promoting the mixing of the HCl gas and the NH₃ gas into the flue gas 12. The mixing promoting auxiliary members 85 are a plurality of plate-shaped members extending in a direction perpendicular to ribs 86 for connecting between the plurality of guide vanes 84. By providing the ribs 86 for connecting between the guide vanes 84 with the mixing promoting auxiliary members 85, the gas flow of the flue gas 12 can be disturbed. Therefore, even when the mixing of the HCl gas and the NH₃ gas in the flue gas 12 is not sufficient in the mixer 71, the mixing of the HCl gas and the NH₃ gas in the flue gas 12 can be promoted on the upstream side of the reduction-denitration device 18. Thus, it is possible to suppress concentration unevenness of the HCl gas and the NH₃ gas in the flue gas 12, thereby improving the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Ninth Embodiment

An air pollution control device according to the ninth embodiment of the present invention will be described with reference to the drawings. FIG. 28 is a diagram showing the configuration of the air pollution control device according to the ninth embodiment of the present invention in a simplified manner. Note that since the air pollution control device according to the present embodiment has a similar configuration to the air pollution control devices according to the first to eighth embodiments, identical elements will be denoted by the same reference numerals and the redundant description will be omitted.

As shown in FIG. 28, an air pollution control device 90 according to the present embodiment includes ammonia (NH₃) gas jet means 92, provided between the NH₄Cl solution supply means 16 and the reduction-denitration device 18, for supplying ammonia (NH₃) gas 91 into the flue gas duct 13 as a reducing agent. The NH₃ gas jet means 92 is composed of an NH₃ gas supply unit 93 for storing the NH₃ gas 91, an ammonia (NH₃) gas feeding pathway 94 for feeding the NH₃ gas 91 to the flue gas duct 13, and a jet nozzle 95 for jetting the NH₃ gas 91 into the flue gas duct 13. Moreover, a jetted amount of the NH₃ gas 91 jetted from the jet nozzle 95 is adjusted by a valve V4. Unlike the droplets such as those of the NH₄Cl solution 14, the NH₃ gas 91 does not cause a damage such as a damage to the flue gas duct 13 even if it collides against the flue gas duct 13. Thus, the NH₃ gas 91 can be jetted also to a wall surface region of the flue gas duct 13. As a result, it is possible to increase the NH₃ concentration in the low-concentration region near the wall of the flue gas duct 13, and it is therefore possible to suppress the concentration unevenness of the HCl gas and the NH₃ gas in the flue gas 12.

Moreover, the position at which the NH₃ gas 91 is supplied into the flue gas duct 13 from the jet nozzle 95 is preferably at 1 m or more downstream side of the spray position of the NH₄Cl solution 14. This is for preventing the droplets of the NH₄Cl solution 14 from colliding against the jet nozzle 95.

Thus, according to the air pollution control device 90 of the present embodiment, by jetting the NH₃ gas 91 into the flue gas duct 13 by the NH₃ gas jet means 92 after spraying the NH₄Cl solution 14 into the flue gas duct 13, it is possible to increase the NH₃ concentration in the low-concentration region near the wall of the flue gas duct 13. Thus, it is possible to deal with the concentration unevenness of the HCl gas and the NH₃ gas in the flue gas 12, and also to maintain the Hg oxidation ability and improve the NOx reducing ability in the reduction-denitration device 18.

<Control for Jetted Amount of NH₃ Gas>

The flowmeter 51 for measuring a flow rate of the flue gas 12 is provided on the upstream side of the spray nozzle 15, and a flow rate of the flue gas 12 is measured. Based on the value of the flow rate of the flue gas 12 measured by the flowmeter 51, the control device 52 can adjust the flow rate, angle, initial velocity, and the like of the NH₃ gas 91 jetted from the jet nozzle 95.

Thus, when a balance between the NOx concentration and the Hg concentration in the flue gas 12 discharged from a combustion facility such as the boiler 11 is different from the ordinal balance to have a higher NOx concentration and the necessary amount of NH₃ therefore cannot be supplied only by spraying the NH₄Cl solution 14 into the flue gas duct 13, it is possible to supply, to the flue gas 12, the necessary amount of NH₃ gas for reducing NOx while reducing unevenness of the concentration distributions of the HCl gas and the NH₃ gas supplied into the flue gas 12 within the flue gas duct 13 by jetting the NH₃ gas 91 into the flue gas duct 13 from the jet nozzle 95. As a result, it is possible to deal with the concentration unevenness of the HCl gas and the NH₃ gas in the flue gas 12 and also to improve the Hg oxidation ability and the NOx reducing ability in the reduction-denitration device 18.

Moreover, the supply amount of the NH₃ gas 91 supplied from the NH₃ gas supply unit 93 may be controlled by using the value of the NOx concentration meter 53.

Moreover, although only the NH₃ gas supply unit 93 is provided and the NH₃ gas 91 is thereby supplied into the flue gas duct 13 in the air pollution control device 90 according to the present embodiment, the present invention is not limited thereto. Instead of the NH₃ gas supply unit 93, a hydrogen chloride (HCl) gas supply unit for supplying a hydrogen chloride (HCl) gas into the flue gas duct 13 as an oxidizing gas may be provided, thereby supplying the HCl gas into the flue gas duct 13. As a result, it is possible to supply, to the flue gas 12, the necessary amount of HCl gas for oxidizing Hg. Moreover, based on the flow velocity of the flue gas 12 measured by the flowmeter 51, it is possible to adjust the sprayed amount, spray angle, and initial velocity of the HCl gas supplied from the HCl gas supply unit.

Furthermore, both of the NH₃ gas supply unit 93 and the HCl gas supply unit may be provided. Based on the flow velocity of the flue gas 12 measured by the flowmeter 51, it is possible to adjust the sprayed amounts, spray angles, and initial velocities of the NH₃ gas 91 and the HCl gas supplied from the NH₃ gas supply unit 93 and the HCl gas supply unit. With this configuration, the NH₃ gas and the HCl gas are separately supplied to the flue gas 12, and it is therefore possible to appropriately deal with a case where the NOx or Hg concentration in the flue gas 12 varies.

The oxidation auxiliary agent used for the oxidizing gas is not limited to HCl, and hydrogen halide other than HCl, such as hydrogen bromide (HBr) or hydrogen iodide (HI), may be used as oxidizing gas.

INDUSTRIAL APPLICABILITY

As described above, the air pollution control device according to the present invention can supply the NH₄Cl solution sprayed into the flue gas duct while preventing the NH₄Cl solution from being adhered to the inner wall of the flue gas duct, and therefore promote the mixing of the HCl gas and the NH₃ gas, which are generated from droplets of the NH₄Cl solution, with the flue gas. Thus, the air pollution control device according to the present invention is suitable for use as an air pollution control device for reducing the amounts of Hg and NOx in the flue gas.

REFERENCE SIGNS LIST

-   -   10, 70, 90 air pollution control device     -   11 boiler     -   12 flue gas     -   13 flue gas duct     -   14 ammonium chloride (NH₄Cl) solution     -   15, 61 spray nozzle     -   16 ammonium chloride (NH₄Cl) solution supply means         (reduction-oxidation auxiliary agent supply means)     -   18 reduction-denitration device (reduction-denitration means)     -   19 heat exchanger (air heater)     -   20 precipitator     -   21 limestone-gypsum slurry     -   22 wet desulfurization device     -   25 ammonium chloride (NH₄Cl) solution supply pipe     -   26, 33, 47 air     -   27, 34 air supply pipe     -   28 ammonium chloride (NH₄Cl) solution tank     -   31, 36 air supply unit     -   32 blowing pipe     -   35 jet hole     -   37 gap     -   38-1 to 38-3 denitration catalyst layer     -   39 current plate     -   41 device body     -   42 absorbent feed line     -   43 nozzle     -   44 purged gas     -   45 stack     -   46 water     -   48 dehydrator     -   49 gypsum     -   51 flowmeter     -   52 control device     -   53 NOx concentration meter     -   54-1, 54-2 mercury (Hg) concentration meter     -   55 bottom portion     -   56 oxidation-reduction potential measurement control device (ORP         controller)     -   71 mixer (mixing means)     -   72 swirling flow inductive member     -   73 first swirling flow inductive plate     -   74 second swirling flow inductive plate     -   75 intermediate member (connecting portion)     -   76 lower support plate     -   77 upper support plate     -   81 protruding member     -   82 narrowed portion     -   83 narrowed member     -   84 guide vane     -   85 mixing promoting auxiliary member     -   86 rib     -   91 ammonia (NH₃) gas     -   92 ammonia (NH₃) gas jet means     -   93 NH₃ gas supply unit     -   94 ammonia (NH₃) gas feeding pathway     -   95 jet nozzle     -   V1 to V4 valve 

1. An air pollution control device for reducing amounts of a nitrogen oxide and mercury contained in a flue gas from a boiler, the device comprising: a reduction-oxidation auxiliary agent supply unit for spraying in a liquid state a reduction-oxidation auxiliary agent that produces an oxidizing gas and a reducing gas upon gasification thereof, into a flue gas duct at a downstream of the boiler by a plurality of spray nozzles; a reduction-denitration unit including a denitration catalyst for reducing a nitrogen oxide in the flue gas with the reducing gas and for oxidizing mercury under coexistence with the oxidizing gas; and a wet desulfurization unit for reducing the amount of mercury oxidized in the reduction-denitration unit using an alkali absorbent, wherein the spray nozzles supply the reduction-oxidation auxiliary agent into the flue gas duct so as to prevent the reduction-oxidation auxiliary agent from being adhered to an inner wall of the flue gas duct.
 2. The air pollution control device according to claim 1, wherein, based on a distance 1 required for droplet gasification, which is obtained at least from a gas flow velocity, a droplet initial velocity, a droplet diameter, a flue gas temperature and a droplet temperature, and a jet angle α, the spray nozzles are disposed so that a shortest distance x from a nozzle hole of the spray nozzle to the inner wall of the flue gas duct satisfies the following expression: x>1×sin α  (1).
 3. The air pollution control device according to claim 1, wherein the reduction-oxidation auxiliary agent is ammonium chloride.
 4. The air pollution control device according to claim 1, wherein a nozzle hole of the spray nozzle is provided at a position 0.5 m or longer away from a wall surface of the flue gas duct.
 5. The air pollution control device according to claim 1, wherein nozzle holes of the plurality of spray nozzles are disposed so as to satisfy the following expression: a≦b/5  (2) where a denotes a distance between the spray nozzles, and b denotes a long side length out of lengths of a cross-section of the flue gas duct.
 6. The air pollution control device according to claim 1, wherein the spray nozzle has a plurality of nozzle holes for spraying the reduction-oxidation auxiliary agent.
 7. The air pollution control device according to claim 6, wherein a distance between the nozzle holes is 0.3 m or shorter.
 8. The air pollution control device according to claim 1, wherein respective sprayed amounts from the spray nozzles can be changed.
 9. The air pollution control device according to claim 1, further comprising one of or both of an ammonia gas supply unit for supplying an ammonia gas into the flue gas duct and a hydrogen chloride gas supply unit for supplying a hydrogen chloride gas into the flue gas duct, which are provided between the reduction-oxidation auxiliary agent supply unit and the reduction-denitration unit.
 10. A flue gas mercury reducing method for reducing amounts of a nitrogen oxide and mercury contained in a flue gas from a boiler, the method comprising: a reduction-oxidation auxiliary agent supplying step of spraying, in a liquid state, a reduction-oxidation auxiliary agent, which produces an oxidizing gas and a reducing gas upon gasification thereof, into a flue gas duct of the boiler by a plurality of spray nozzles; a reduction-denitration treatment step of reducing a nitrogen oxide in the flue gas with the reducing gas and for oxidizing mercury under coexistence with the oxidizing gas using a denitration catalyst; and a wet desulfurization step of reducing an amount of mercury oxidized in the reduction-denitration treatment step using an alkali absorbent, wherein the spray nozzles supply the reduction-oxidation auxiliary agent into the flue gas duct so as to prevent the reduction-oxidation auxiliary agent from being adhered to an inner wall of the flue gas duct.
 11. The flue gas mercury reducing method according to claim 10, wherein based on a distance 1 required for droplet gasification, which is obtained at least from a gas flow velocity, a droplet initial velocity, a droplet diameter, a flue gas temperature, and a droplet temperature, and a jet angle α, the spray nozzles are disposed so that a shortest distance x from the nozzle hole of the spray nozzle to the inner wall of the flue gas duct satisfies the following expression: x>1×sin α  (3).
 12. The flue gas mercury reducing method according to claim 10, wherein the reduction-oxidation auxiliary agent is ammonium chloride.
 13. The flue gas mercury reducing method according to claim 1, wherein a nozzle hole of the spray nozzle is provided at a position 0.5 m or longer away from a wall surface of the flue gas duct.
 14. The flue gas mercury reducing method according to claim 1, wherein nozzle holes of the plurality of spray nozzles are disposed so as to satisfy the following expression: a≦5 b/5  (4) where a denotes a distance between the spray nozzles, and b denotes a long side length out of lengths of a cross-section of the flue gas duct.
 15. The flue gas mercury reducing method according to claim 1, further comprising a flow rate measurement step of measuring a flow velocity of the flue gas on an upstream side of a supply position at which the reduction-oxidation auxiliary agent is supplied, wherein a sprayed amount, a spray angle, and an initial velocity of the reduction-oxidation auxiliary agent are adjusted based on the measured flow velocity of the flue gas.
 16. The flue gas mercury reducing method according to claim 1, further comprising: a nitrogen oxide concentration measurement step of measuring a nitrogen oxide concentration in the flue gas on a pre-step side of the reduction-denitration treatment step; and a mercury concentration measurement step of measuring a mercury concentration in the flue gas on a post-step side of the reduction-denitration treatment step, wherein based on one of or both of the nitrogen oxide concentration in the flue gas obtained by the nitrogen oxide concentration measurement step and the mercury concentration in the flue gas obtained by the mercury concentration measurement step, a supply amount of the reduction-oxidation auxiliary agent to be supplied in the reduction-oxidation auxiliary agent supplying step is adjusted.
 17. The flue gas mercury reducing method according to claim 15, further comprising one of or both of an ammonia gas supplying step of supplying an ammonia gas into the flue gas duct and a hydrogen chloride gas supplying step of supplying a hydrogen chloride gas into the flue gas duct between the reduction-oxidation auxiliary agent supplying step and the reduction-denitration treatment step, wherein a sprayed amount, a spray angle, and an initial velocity of one of or both of the ammonia gas supplied by the ammonia gas supplying step and the hydrogen chloride gas supplied by the hydrogen chloride gas supplying step are adjusted, based on the flow velocity of the flue gas measured by the flow rate measurement step. 